The present invention relates to light emitting diodes and more particularly to arrays of light emitting diodes.
Arrays of light emitting diodes (LEDs) may be employed in a variety of high flux (optical energy/unit time) applications such as street lighting, traffic signals, and liquid crystal display back-lighting. It is advantageous in such applications to increase the flux provided per unit area of the LED array. Such an increase in flux per unit area can in principle be accomplished by decreasing the spacing between LEDs in the array (thus increasing the number of LEDs per unit area of the array) and/or by increasing the flux provided by the individual LEDs. However, either approach to increasing the flux per unit area of an LED array also typically increases the amount of heat that must be dissipated per unit area of the array to avoid significantly degrading the performance of the LEDs.
One conventional type of LED array includes LED lamps which each comprise a die attached to a metal leadframe within a molded or cast plastic body. The two metal leads of each lamp are typically soldered to conductive traces on a common printed circuit board of conventional design. The space taken up by the lamps"" plastic bodies and metal leads limits the packing density of lamps in the array. In addition, such lamps typically cannot dissipate much heat, since heat generated in the lamps is conducted away primarily through the leads and the conductive traces. Consequently, the lamps cannot be driven by high current to produce high flux.
In another conventional type of LED array LED dice are attached with silver filled epoxy to the surface of a conventional printed circuit board. Typically, the LED dice are electrically connected to conductive traces on the printed circuit board with wire bonds which limit how closely the dice may be spaced. In addition, the conventional printed circuit board to which the dice are attached is typically not effective at spreading the heat generated by the LED dice. This limits the maximum operating current of the LED dice and thus the maximum flux provided by an individual LED die.
What is needed is a high flux LED array that does not suffer from the drawbacks of prior art LED arrays.
An LED array in accordance with the present invention includes a metal substrate, a dielectric layer disposed above the metal substrate, and a plurality of electrically conductive traces disposed on the dielectric layer. A plurality of vias pass through the dielectric layer. The LED array also includes a plurality of LEDs, each of which is disposed above a corresponding one of said vias and each of which includes a first electrical contact and a second electrical contact electrically coupled to separate ones of the electrically conductive traces. Each of the vias contains a thermally conductive material in thermal contact with the metal substrate and in thermal contact with the corresponding LED. The thermally conductive material may include, for example, a solder material.
In some embodiments, the thermally conductive material in a via is in direct physical contact with the metal substrate, in direct physical contact with the corresponding LED, or in direct physical contact with both the metal substrate and the LED. Such direct physical contact is sufficient but not necessary to establish thermal contact.
The thermally conductive material in a via provides an advantageously low thermal resistance path for heat to flow from an LED disposed above the via to the metal substrate, which then effectively conducts the heat away. Consequently, LEDs in arrays in accordance with the present invention may be operated at higher currents and may be spaced closer together than is conventional without raising their temperatures to levels that degrade their performance. Hence, the inventive LED arrays may provide more flux per unit area than is provided by conventional LED arrays.
In some embodiments, a submount is disposed between an LED and the corresponding via and in direct physical contact with the thermally conductive material in the via. Use of such a submount allows incorporation of additional circuitry and testing of the LED prior to installation in the array.